1. Field of the Invention
This invention relates to a printed circuit board, and more particularly to a printed circuit board having excellent adhesion property between conductor formed through an additive process and resin insulating layer and conductance resistance.
2. Description of the Related Art
Recently, there has been increased miniaturization and high-speed performance of electronic equipment with the progress in the electronics industry, and consequently it is necessary to have high densification and reliability through fine pattern against printed circuit board and printed wiring substrate mounting LSI.
Lately, an additive process in which an adhesive is applied onto a surface of a substrate to form a resin insulating layer, and the surface of the resin insulating layer is roughened and then subjected to an electroless plating to form a conductor is noticed as another method of forming the conductor onto the printed wiring substrate.
According to this method, the conductor is formed by electroless plating after the formation of the resist, so that it has a merit that conductor wiring having high density and pattern accuracy can be produced with a low cost as compared with an etched foil process conducting the formation of the pattern through etching (which is known as a subtractive process).
In the additive process, there have hitherto been known a method in which fine unevenness is formed on the surface side of the resin insulating layer for the formation of the conductor through chemical etching as a means for improving the adhesion property between conductor and resin insulating layer (hereinafter referred to as peel strength). According to this method, the unevenness of the surface of the resin insulating layer is subjected to a metal plating, such as copper plating, whereby the peel strength can be improved through an anchor effect based on the unevenness. Such an improvement of the peel strength through the anchor effect is generally carried out by enlarging the braking surface or by increasing the strength of the conductor metal or the resin insulating layer.
In recent additive type printed circuit boards requiring the formation of wiring with high density and pattern accuracy, it is required to make small anchors formed by surface roughening of the resin insulating layer in order to precisely form the fine pattern of the resist. Therefore, this conventional technique causes a problem that, as the anchor becomes small, the breaking area is also small, and hence the peel strength considerably lowered.
In order to solve this problem, there is a method of increasing the strength of the resin, but the breakage of the conductor metal portion becomes a problem.
When the modulus of elasticity and yield point of the conductor metal are low, the metal filled in the anchor is plastically deformed through a very small external force to be removed from the anchor, and the conductor is peeled off from the resin insulating layer.
Further, when the tensile strength of the conductor metal is low, the metal filled in the anchor is broken by a very small external force, and the conductor is peeled off from the resin insulating layer.
Particularly, the above phenomenon is conspicuous in copper, which has a generally low modulus of elasticity and yield point or tensile strength.